The present invention relates to an improved magnetic encoder, and more particularly relates to production of a magnetic encoder adapted for detection of angular displacement, linear displacement and displacement speed of a mobile body in a drive system for robots or NC devices.
Conventional magnetic encoders are roughly classified into two typical groups each of which includes a rotary spindle mechanically coupled to the drive system. Such a rotary spindle is generally mounted to a housing via suitable bearings. In the case of the first group, a drum-type magnetic recording medium is coaxially coupled to the rotary spindle outside the housing and a magnetized pattern is formed on the peripheral face of the drum. A magnetic detection head is arranged facing the magnetized pattern on the drum while leaving a prescribed clearance. In the case of the second group, a disc-type magnetic recording medium is again coaxially coupled to the rotary spindle outside the housing and a magnetized pattern is formed on the end face of the disc along the periphery. A magnetic detection head is arranged facing the magnetized pattern on the disc while leaving a prescribed clearance.
In either group, as a recording medium rotates with the rotary spindle, a magnetic field applied to the detection head varies as the magnetized pattern on the recording medium moves and, in response such variation in the magnetic field, the detection head generates a series of output signals representative of the magnetic pattern on the recording medium for detection of angular displacement, linear displacement or displacement of the mobile body in the drive system connected to the rotary spindle.
Since the recording medium is prepared separate from the rotary spindle, both have to be mechanically coupled to one another during production, and such coupling process requires much labour and time. In addition, the degree of preciseness of this coupling process poses a serious influence on the functional reliability of the magnetic encoder. That is, for correct detection of the displacement or displacement speed of the mobile body, the clearance between the detection head and the magnetized pattern on the recording medium has to be maintained as constant as possible during rotation of the recording medium. In practice, however, the clearance inevitably fluctuates due to production errors mainly caused by a low degree of preciseness in coupling. It is extremely difficult to achieve the coupling process with a sufficiently high degree of preciseness.
In order to remove such drawbacks, it was proposed by the inventors of the present invention in a copending U.S. patent application Ser. No. 168,160 filed on Mar. 15, 1988, now U.S. Pat. No. 4,851,752 to form a magnetized pattern on the peripheral face of a rotary spindle made of a magnetic material. By using the rotary spindle itself as a recording medium, it is no longer necessary to mechanically couple a recording medium to the rotary spindle and the functional reliability of the magnetic encoder is much improved due to absence of a coupling process of a low degree a preciseness. However, such a peripheral-magnetization type magnetic encoder requires arrangement of a detection head in a face-to-face relationship to the peripheral face of the rotary spindle. This arrangement makes it difficult to achieve compactness of the magnetic encoder, which is nowadays generally demanded in the field of electric and electronic devices.